From the Designer help:
The %html.core.attrs; entity lists the attributes for core html elements. This entity is used in the <actionhotspot>, <area>, <button>, <computedtext>, <embeddedfileuploadcontrol>, <field>, <frame>, <frameset>, <horizrule>, <javaapplet>, <table>, <tablecell>, and elements.
Is there a reason why <picture> is not listed here? Pictures most definitely have those attributes (including one more: the htmlname). You can export a design element or document that has a picture with those attributes specified and you will see them in the stream as expected. You cannot, however, import an element with those attributes set. I'm guessing somebody at IBM just overlooked this, but does anybody have any insight into this?